1. Field of the Invention
This invention relates to a medical device, and more particularly, to a locking assembly for the plunger of a syringe.
2. Background of the Art
Retractable syringe technology has been a rapidly advancing art fueled by concern with the threat of the disease AIDS in the general population and the belief that a major cause of this problem is a result of the sharing and reuse of hypodermic syringes by IV drug users. An important collateral problem is the problem of accidental needle sticks among health care workers and sanitation employees of hospitals from syringe needles that had been used on infected patients.
Many of the known single use retractable syringes have mechanisms which permit retraction of the used needle into the syringe body or plunger of the syringe after use. Many of the known devices employ some kind of triggering mechanism by which the syringe needle is automatically retracted into the syringe body when the plunger is depressed at the end of an injection cycle. Exemplary of one type of such automatically retractable syringe is found in my own patent application, Ser. No. 08/125,292, now U.S. Pat. No. 5,385,551 although there are a number of other devices with at least a portion of a retraction mechanism in a front end portion of a syringe which retract automatically when the plunger is depressed to complete an injection. In a number of such cases, the hollow plunger is sealed with a plug member or a diaphragm at the front of the plunger which is dislodged, pierced or broken when the plunger is depressed to inject the last of the injection fluid from the plunger and a needle holding element is driven up into the hollow plunger so that the sharp needle is no longer exposed.
It has been found possible to employ a catch on the front of a syringe plunger which hooks a projection on the inside of the syringe body at about the time a retraction mechanism is being triggered by continued depression of the plunger after the injection is completed. It is highly desirable to prevent the plunger from being withdrawn from the syringe body after the needle is retracted so that it is very difficult for a subsequent user to take the syringe apart and reassembly the components for reuse. This is true even for syringes which do not have a retractable needle. If the plunger cannot be pulled back after one use, then it cannot be reused, although if the needle is not retracted, there remains a risk of needle sticks.
It has been found that bubbles of air tend to accumulate under any kind of catch on the front tip end of a syringe plunger during the process of filling with injection fluid a syringe having a locking plunger. Some air inevitably remains in the variable fluid chamber of the syringe when the needle is placed in a vial of fluid and the plunger is withdrawn to create a vacuum which will draw injection fluid into the injection chamber. This entrained air generally exhibits itself as bubbles which are visible through the clear walls of the typical syringe.
The usual procedure to deal with these bubbles is to hold the syringe vertical while lightly depressing the plunger to expel the undesirable air bubbles while adjusting the dose by expelling a small quantity of the injection fluid through the needle. This operation is sometimes accompanied by tapping the wall to release the bubbles which tend to adhere by surface tension, but are usually easily dislodged. However, it is found that when the tip of the syringe plunger is equipped with a catch which may be visualized in cross section as a form of arrowhead, a transverse surface is present which becomes an ideal place for these bubbles to accumulate.
It might be possible to alleviate this problem if the usual seal on the head of the plunger which is designed for sliding contact with the walls of the syringe body is abutted up against the catch on the tip of the syringe. While this has the prospect of eliminating the accumulation of bubbles under the catch, it makes it very difficult to create a locking mechanism because of interference between the seal and the cooperating portion of the locking mechanism on the syringe body. In addition, the seal may undesirably increase the amount of force required to depress the plunger in order to move it to the locking position and interfere with smooth operation of any retraction mechanism.
Consequently, there is a need for a syringe plunger locking assembly for use with a single use syringe having a syringe plunger with a catch wherein bubbles of air are prevented from accumulating behind the catch or are easily released therefrom, without interfering with the smooth depression of the plunger. Such a locking mechanism should not increase the force necessary to depress the plunger and trigger a retraction mechanism and without deteriorating the smooth operation and the extreme degree of reliability necessary in a syringe with a retraction mechanism.